This invention relates to a hydraulic brake system composed of a pedal-actuated brake power booster connected with the master cylinder. The system comprises a booster piston and a booster chamber wherein an auxiliary pressure proportional to the pedal force develops, and further comprising an auxiliary-pressure supply system and wheel brakes connected to the master cylinder.
A device having the above features is disclosed in German printed and published patent application No. 31 08 908.9. In the disclosed brake power booster, a master cylinder piston is located in a first cylinder bore, forming one component part with a booster piston. A master cylinder piston and booster piston are interconnected by way of a tappet. An end surface of the booster piston close to the pedal confines a booster chamber formed in the housing and is disposed coaxially to a pedal-actuatable piston rod, at which, in turn, there is articulated a lever assembly for the actuation of a brake valve. By application of force on the piston rod and by actuation of the brake valve, it is possible to introduce a pressure proportional to the actuating force into the booster chamber, with the pressure displacing the booster piston and the master cylinder piston connected to the booster piston in the direction of actuation of the power booster, whereby a corresponding pressure is built up in the working chamber of the master cylinder. Connected to the working chamber of the master cylinder are wheel brakes which are thereby pressurized wherein the vehicle slows down correspondingly. On pressurization of the booster chamber, the piston rod coupled to the brake pedal is likewise applied by pressure, so that a corresponding reaction force can be felt at the pedal providing the vehicle driver with feedback regarding the intensity of braking initiated by him.
Vehicles having a high permissible overall weight need a high boosting factor so as to keep the pedal force and the pedal travel within desired limits. In the event of booster failure, the result is undesirably or even inadmissibly high pedal forces and short pedal travels. It is, therefore, an object of the present invention to provide a brake system of the type described which operates with a reduced effective master brake cylinder surface in the event of booster failure.